Method of synthesizing high surface area unagglomerated noble metal pyrochlore compounds

ABSTRACT

A method of preparing electrically conductive pyrochlore compounds of high surface area and unagglomerated form, having the formula 
     
         A.sub.2 [B.sub.2-x A.sub.x ]O.sub.7-y 
    
     is disclosed wherein A is selected from lead, bismuth and mixtures thereof, B is selected from ruthenium, iridium and mixtures thereof, 0&lt;x≦1.0 and 0≦y≦1. The method involves (1) synthesizing the pyrochlores in an aqueous alkaline reaction medium having a pH of at least about 12.0 and in the presence of an oxygen source (2) displacing the reaction medium with water, (3) atomizing the resultant slurry and (4) freeze drying the resultant product. The pyrochlore compounds thus prepared have a variety of applications including use as oxygen electrodes in electro-chemical devices.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to a method of preparing pyrochlorestructure compounds. More particularly, the present invention isdirected to a method of preparing lead-rich and bismuth-rich ruthenateand iridate pyrochlores in an alkaline reaction medium.

2. Description of Relevant Art

A number of electrochemical devices have been developed for producingelectrical energy by electrochemical reaction and, conversely, forconsuming electrical energy to effect electrochemical reactions. Manydevices rely upon a reaction involving oxygen (or air) as part of themechanism to accomplish the desired result. For example, such devicesmay contain oxygen electrodes, which are oxygen reducing cathodes, inwhich oxygen is catalytically electroreduced. Alternatively, suchdevices may contain oxygen electrodes which catalyze the evolution ofoxygen from water. In general, these electrodes are known in the art asoxygen electrodes. Thus, metal-oxygen batteries, metal-air batteries,fuel cells, electrolyzers, metal electrowinning devices, etc., are amongthe well-known electrochemical devices which may contain oxygenelectrodes. Typically, such devices contain electrocatalyst materials atone or more of their electrodes. For example, precious metals such asplatinum (on carbon support) and silver (on carbon and other supports)are frequently employed as electrocatalysts.

Various electrocatalytic alloys, compounds and compound mixtures havebeen developed to enable such electrochemical devices to achieve moredesirable systems. For example, U.S. Pat. No. 3,536,533 (Kitamura)describes the use of an alloy of gold, silver, palladium and at leastone element selected from the group consisting of platinum, rhodium andruthenium as a fuel cell electrode electrocatalyst. U.S. Pat. No.3,305,402 (Jones et al) describes the use of a combination of platinumand ruthenium oxides as an electrocatalyst. However, both Kitamura andJones et al describe these catalysts as fuel cell anode (or fueloxidation) catalysts. In addition, O'Grady et al, Technical Report No.37, "Ruthenium Oxide Catalysts For The Oxygen Electrode", Contract No.,N0014-67-A-0404-0006 (AD-779-899) Office of Naval Research, May 1974(National Technical Information Service) describes the use of rutheniumoxide as an electrochemical catalyst for both the generation and thereduction of oxygen. Also, U.S. Pat. No. 3,405,010 (Kordesch et al)teaches that spinel type electrode catalysts produces better activationof the electrode and improved electrolyte repellency of the electrode bythe inclusion of ruthenium.

Thus, the foregoing prior art describes a variety of electrodes,including those which utilize iridium and/or ruthenium-containingcatalysts. However, none of the references teaches or renders obviousthe bismuth-rich and lead-rich pyrochlore compounds described herein,much less the particular method of preparation claimed herein.

Heretofore, many pyrochlore compounds such as the pyrochlore compoundsPb₂ Ru₂ O_(7-y) (lattice parameter of 10.253 Å), Pb₂ Ir₂ O_(7-y)(lattice parameter of 10.271 Å), Bi₂ Ir₂ O_(7-y), Bi₂ Rh₂ O_(7-y), Pb₂Rh₂ O_(7-y), Pb₂ Pt₂ O_(7-y), Cd₂ Re₂ O_(7-y) (commonly referred to aslead ruthenate, lead iridate, bismuth iridate, bismuth rhodate, leadrhodate, lead platinate and cadmium rhenate, respectively,) and similarcompounds have been known. For example, Longo, Raccah and Goodenough,Mat. Res. Bull., Vol. 4, pp. 191-202 (1969), described the compounds Pb₂Ru₂ O_(7-y) and Pb₂ Ir₂ O_(7-y) and their preparation at temperatures inexcess of 700° C. Sleight, Mat. Res. Bull., Vol. 6, p. 775 (1971) hasalso described the compounds Pb₂ Ru₂ O_(7-y) and Pb₂ Ir₂ O.sub. 7-y(including the pyrochlore compound Pb₂ Ru₂ O₆.5 having a latticeparameter of 10.271 Å) and their preparation at 700° C. and 3000atmospheres. U.S. Pat. No. 3,682,840 (Van Loan) describes thepreparation of lead ruthenate at temperatures of 800° C. and higher.None of these references teach the existance of the lead-rich orbismuth-rich compounds made by the present invention or that suchcompounds can be prepared as claimed herein.

U.S. Pat. Nos. 3,769,382 (Kuo et al) and 3,951,672 (Langley et al)discloses a variety of techniques for preparing lead ruthenate and leadiridate at temperatures of at least about 600° C., and preferably athigher temperatures. However, each reference fails to recognize that thelead-rich pyrochlores used in the present invention are obtained atgenerally lower temperatures or that such pyrochlores have improvedphysical properties. Further, both references fail to teach the presentmethod of preparing lead-rich and bismuth-rich pyrochlore compounds.

Bouchard and Gillson, Mat. Res. Bull., Vol. 6, pp. 669-680 (1971)describe the preparation and properties of Bi₂ Ru₂ O₇ and Bi₂ Ir₂ O₇,including the high conductivity and small Seebeck coefficients of eachcompound. However, there is no teaching that these compounds are usefulelectrocatalysts in electrochemical devices. Derwent's Basic AbstractJournal, Section E, Chemdoc, Week No. Y25, Abstract No. 320 (Aug. 17,1977), Derwent Accession No. 44866Y/25 describes electrodes forelectrolysis of alkaline and carbonate solutions which comprisenickelplated steel strips coated with high conductivity layerscontaining Cd₂ Re₂ O₇, Pb₂ Re₂ O_(7-y) or Ni₂ Re₂ O₇. These compoundsare prepared by impregnating perrhenic acid and a metal nitrate such asCd nitrate onto a nickel strip and baking at 350° C. However, thesecompounds are all rhenates rather than ruthenates or iridates and arenot taught to be lead-rich or bismuth-rich compounds prepared by themethod of the present invention. National Bureau of Standards,Washington, D.C., Institute for Mat. Research, Abstract of Rept. No.NNSIR-75-742 (1974) describes the use of mixed oxides as oxygen-reducingelectrocatalysts in acid fuel cells, including the use of bariumruthenate. However, the materials suggested for such electrocatalystsare not the pyrochlore type structure compounds made according to thepresent invention.

The foregoing prior art dealing with the synthesis of electricallyconductive pyrochlore structure oxides teaches synthesis temperatures ofat least 600° C. While elevated temperatures have been considerednecessary to overcome diffusional limitations encountered in solid statereactions, such temperatures result in the formation of sinteredproducts with low surface areas. This is disadvantageous for materialsused in catalytic and electrocatalytic applications since theconcentration of available catalytically active sites is limited.

To conserve energy and maximize surface area, it would be desirable tosynthesize electrically conductive pyrochlore compounds at significantlylower temperatures, e.g. below 300° C. However, the kinetics of solidstate reactions are unfavorably sluggish. Solution syntheses offer onepossible approach to achieving these very low temperature reactions. Forexample Trehoux, Abraham and Thomas, Journal of Solid State Chemistry,Vol. 21, pp. 203-209 (1977) and C.R. Acad. Sc. Paris, t. 281 pp. 379-380(1975) describe the solution preparation of a pyrochlore compound of theformula K₁.14 Bi^(III) ₀.27 [Bi^(III) ₀.27 Bi^(V) ₄.9 ] [O₄.9 OH₁.1 ]OH₀.8. The synthesis is effected by adding a bismuth nitrate solution toa solution of 17% potassium hydroxide containing an excess of potassiumhypochlorite. The reaction is carried out in this medium for 2 hours ina reflux type of apparatus at a temperature slightly higher than 100° C.The synthesis and resulting product are different in many respects fromthe synthesis and product claimed herein. The compound prepared in thecited reference is not an oxide but rather an oxyhydroxide which has asignificant amount of protons incorporated into the bulk structure.Proton nuclear magnetic resonance experiments show that compoundsprepared according to the present invention are oxides which do not havesignificant amounts of protons incorporated into the structure. Thepyrochlore synthesized by Trehoux et al is not a ruthenium or iridiumcontaining compound and, in fact, is believed not to be an electricallyconductive pyrochlore. The potassium hydroxide solution used in theTrehous reference serves not only as a reaction medium, but also as aconstituent in the reaction since potassium is incorporated into the Asite of the pyrochlore. In contrast, the alkali solution employed in thepresent invention is solely a reaction medium with no measurable amountof alkali metal cations incorporated in the pyrochlore compound product.

Morgenstern-Badarau and Michel, Ann. Chim., Vol. 6, pp. 109 et seq.(especially at 109-113) (1971), and C. R. Acad. Sc. Paris, Vol. 271,Seire C pp. 1313-1316 (1970) report the solution preparation ofpyrochlore compounds having the formula Pb₂ Sn₂ O₆ ×H₂ O where 0<×<1.The preparation conditions are strictly defined as follows: equimolarquantities of lead and tin are reacted from solution in the presence ofthe complexing agent nitrilotriacetic acid (NITA) such that theconcentration of [NITA]/[Pb²⁺ ]=2. The pH of the reaction medium isfixed at 11 and the reaction is carried out for several hours at 80° C.The compound prepared by Morgenstern-Badarau et al is a hydrated oxidewhereas materials prepared according to the present invention areoxides. In addition, the pyrochlore prepared by Morgenstern-Badarau etal, while containing lead, is not similar to the lead-rich pyrochloreprepared according to the present invention. Further, the pyrochloreprepared by Morgenstern-Badarau et al is not a ruthenium or iridiumcontaining pyrochlore and is not believed to be electrically conductive.Also, Morgenstern-Badarau et al specifically state that their method ofpreparation forms a solid product containing Pb²⁺. In contrast, thesolid product formed according to the present invention contains amixture of Pb²⁺ and Pb⁴⁺. While the presence of a complexing agent isrequired in the synthesis described in the cited reference, such acomplexing agent is not required according to the present invention.Furthermore, the pH range of the synthesis medium specified in thepresent invention clearly differs from the operable pH range of thecited reference. In fact, the Morgenstern-Badarau and Michel, Ann.Chim., Vol. 6, pp. 109-124 (l971) reference clearly states that no solidproduct compound can be obtained if conditions which are coincident withthose specified for the present invention (pH>13.5, temperature=80° C.,zero concentration of complexing agent) are employed.

More recently, U.S. Pat. Nos. 4,129,525; 4,163,706; 4,176,094;4,192,780; 4,203,871 and 4,225,469 to Horowitz et al form pyrochlorecompounds in which the pyrochlore oxide is precipitated from an alkalinesolution and then separated therefrom by filtration. The filtrate iswashed with water and dried to yield the pyrochlore solids. In U.S. Pat.Nos. 4,124,539 also to Horowitz et al, the precipitate is not recoveredby filtration, but instead, the liquid of the pyrochlore/alkalinesolution suspension is evaporated to dryness and the resulting oxide iswashed in alkali or acetic acid. While the alkaline precipitation mediumis the same as that disclosed in the other Horowitz et al patents, theparticular method of separating the pyrochlore oxide precipitate fromthe precipitation medium claimed herein is markedly different.

Therefore, in summary, there exists a formidable body of prior artdescribing the existence of various pyrochlores, their potential uses(including uses as dielectric materials) and describing various metalsand metal oxides as electrocatalyst materials. Notwithstanding thisprior art, there is no suggestion or teaching that (a) the lead-rich orbismuth-rich pyrochlore compounds made according to the presentinvention exist, or that (b) the present invention may be used to makesuch compounds.

SUMMARY OF THE INVENTION

The present invention is directed to a method of preparing compoundshaving the formula:

    A.sub.2 [B.sub.2-x A.sub.x ]O.sub.7-y                      ( 1)

wherein A is selected from the group consisting of lead, bismuth andmixtures thereof, B is selected from the group consisting of ruthenium,iridium and mixtures thereof, and wherein x is a value such that 0<x≦1.0and y is a value such that 0≦y≦1. More specifically, the presentinvention relates to synthesizing pyrochlore compounds having theformula shown in (1) in an alkaline medium, displacing the alkalinemedium with water, atomizing the resultant slurry and freeze drying theresultant product. When prepared in this manner, there will be formednoble metal pyrochlore compounds of high surface area and reducedagglomeration relative to that obtained in the absence of the atomizingand freeze drying steps. The compounds thus formed have many usesincluding use as oxygen electrodes in electrochemical devices.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the variation of lattice parameter with increasingamounts of lead on B cation sites.

FIG. 2 illustrates the variation of lattice parameter with increasingamounts of bismuth on B cation sites.

FIG. 3 illustrates the variation of oxygen reduction voltage withcurrent density for Pb₂ [Ru₁.42 Pb₀.58 ]O₆.5 prepared conventionally andin accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The compounds made according to the present invention, as represented byformula (1) above, display the pyrochlore crystal structure. Pyrochlorestructure oxides are represented by the general formula A₂ B₂ O₆ O'wherein A and B are metal cations. A detailed description of theircrystallographic structure may be found in Structural InorganicChemistry, Fourth Edition by A. F. Wells, Clarendon Press, Oxford, 1975.Briefly, oxides of this type display a face-centered cubic structurehaving a unit cell dimension of about 10 Å. The B cations areoctahedrally coordinated by oxygen anions (O). The structural frameworkis formed by a three dimensional array of the corner shared octahedra,each sharing corners with six others. This framework has the compositionB₂ O₆. As Wells describes, this framework of octahedra is "based on thediamond net, having large holes which contain the O' and two A atoms,which themselves form a cuprite-like net A₂ O' interpenetrating theoctahedral framework". The octahedra are actually arranged intetrahedral clusters. These clusters of octahedra are then tetrahedrallyarranged so as to form the large holes in the structure described byWells. Each of these large holes may also be defined by fourtetrahedrally arranged puckered, hexagonal rings which are formed by thecorner shared octahedra. The A cations reside in the center of thesepuckered hexagonal rings and are coordinated by the six O anions whichdefine the rings plus two more O' cations at a slightly differentdistance. These O' anions reside at the center of the large holes in theoctahedral framework. It is the O' anions which may be partially ortotally absent, leading to the general pyrochlore oxide formula A₂ B₂O_(7-y) where 0≦y≦1. Thus, the compounds made according to the presentinvention are referred to as pyrochlore compounds, albeit they are notstoichiometric pyrochlores, but rather are lead-rich and/or bismuth-richcompounds of the formula above.

The pyrochlore compounds prepared according to the present inventionexhibit an expanded lattice. While not wishing to be bound by anyparticular theory, it is believed that substitution of the larger leador bismuth for the smaller noble metal cations on the octahedrallycoordinated B-site leads to the considerable enlargement of thepyrochlore unit cell dimension. The relationship between latticeparameter (a_(o)) and the extent of substitution of the octahedrallycoordinated ruthenium or iridium by lead or bismuth is linear and thisrelationship is illustrated for two series of compounds in FIGS. 1 and2. The greater the amount of lead or bismuth on the B-site (as indicatedby the value of x in formula 1), the greater the lattice parameter. (SeeH. S. Horowitz, J. M. Longo and J. T. Lewandowski in Mat. Res. Bull.,16, 489-96 (1981), the entire disclosure of which is incorporated hereinby reference).

In general terms, the present invention involves first reacting A and Bcations to yield a pyrochlore oxide by precipitation of A and B cationsfrom an aqueous solution source of these cations in a liquid alkalinemedium in the presence of an oxygen source at a temperature below about200° C. The synthesis occurs entirely in a solution medium where thereaction kinetics are quite favorable and not so restrictive as is foundin solid state reactions, notwithstanding the low reaction temperatureemployed in the present method. These conditions result in the formationof product in high surface area (i.e. 60-200 m² /g). High surface areais particularly advantageous for a material used in a catalytic orelectrocatalytic application since the concentration of availablecatalytically active sites is maximized.

The aqueous solution source of reactant (A and B) cations is meant bydefinition to include any aqueous solution which will dissolve ionic Aand B cations. This metal cation containing solution may be preparedusing A source materials which include lead nitrate, lead oxide, leadchloride, lead acetate, lead carbonate, lead citrate, lead oxalate,bismuth nitrate, bismuth oxide, bismuth chloride, bismuth oxalate andbismuth oxychloride as well as mixtures thereof. Desirably, the A sourcematerial used in preparing the aqueous solution source of A and Bcations is either a lead source material or a bismuth source material,although, mixtures of each may be used. Preferred A source materials arelead and bismuth nitrates. The B source materials used in preparing theaqueous solution source of A and B cations include ruthenium chloride,ruthenium nitrate, ruthenium nitrosyl nitrate, iridium chloride, iridiumhydroxide and iridium oxalic acid as well as mixtures thereof.Desirably, the B source material is either a ruthenium source or aniridium source, although mixtures thereof may be employed. The preferredB source materials include ruthenium nitrate and iridium chloride.

The aqueous solution source of A and B cations is prepared by dissolvingappropriate amounts of A source material and B source material inaqueous solvent. In some cases water is adequate for the dissolution.When necessary, the A and B source materials may be dissolved in aqueousacid solutions, the acid solutions being just strong enough to cause theA and B source materials to dissolve. Acids such as nitric orhydrochloric may be used, but nitric acid is preferred.

The A source material and B source material are dissolved in relativeamounts so as to achieve, in general, an initial reactant A to B ionratio of at least about 1.0:1.0. Desirably, this ratio is within therange of about 1.05:1.0 to about 10.0:l.0. In the preferred embodiments,the A to B ion ratio is in the range of about 1.2:1.0 to about 5.0:1.0.As a practical matter, the reactants may be used in an A to B ion ratioappreciably higher than the ratio of A to B in the final pyrochloreproduct.

Preparation of the aqueous solution source of A and B cations in themanner just described assures atomic scale mixing of these cations andthereby provide favorably kinetics for the low temperature, solutionmedium synthesis that follows.

The liquid alkaline medium is meant by definition to include any liquidalkaline medium which will promote reaction between the A ions and Bions from the mentioned aqueous solution source of A and B cations andwill effect the precipitation of the desired pyrochlore structure. Theliquid alkaline medium may be any which satisfies this definition andincludes aqueous basic solutions of alkali metal hydroxides. Thus, theliquid alkaline medium may desirably be an aqueous basic solutioncontaining a base selected from the group consisting of sodiumhydroxide, rubidium hydroxide, cesium hydroxide, potassium hydroxide andmixtures thereof. Desirably, sufficient base is included so as to form aliquid alkaline medium having a pH of at least about 12.0. Preferably,sufficient base is employed so as to produce a liquid alkaline mediumhaving a pH of between about 13 and 14. Exact amounts of base materialneed not be specified since pH determination is within the purview ofthe artisan.

It is also helpful, although not necessary, to saturate the alkalinereaction medium with one or more of the reactant cations (and especiallywith the most alkali soluble cation reactant) prior to combining theaqueous solution source of A and B cations with the alkaline reactionmedium. This avoids large discrepancies between cation ratios in thereacted product and in the initial reactant mixture due to possiblesolubility in the alkaline reaction medium of one or more of thereactant cations.

The alkaline medium acts solely as a reaction medium and not as aconstituent in the reaction. This is supported in that the pyrochloresprepared according to this invention show less than 0.02% (by weight)alkali metal cation as measured by atomic absorption.

The oxygen source is meant to include any source which will provide theoxygen needed to form the pyrochlore compound. The oxygen source may beany of the A source material, the M source material, the alkaline liquidmedium or combinations thereof. Alternatively or additionally, theoxygen source may be or include independent oxygen contributingmaterial, e.g., bubbled oxygen or oxygen-containing salts or otheradditives. In any event, an essential aspect of the present invention isthe inclusion of adequate oxygen to permit the formation of thepyrochlore structure.

No criticality exists as to whether the aqueous solution source of A andB cations is added to the alkaline medium or whether the alkaline mediumis added to the aqueous solution source of reactant cations. However,the former is usually practiced to insure that all of the cationscontact an excess of alkaline medium. In general, at least about 1.0liter of liquid alkaline medium is used per sum total mole of metalcation reactant. As mentioned, the reaction should be carried out attemperatures below about 200° C. Desirably, the reaction temperature iswithin the range of about 10° to about 100° C. Preferably, the reactionis carried out at temperatures within the range of about 50° to about80° C.

During the reaction period the alkaline medium may be replaced withfresh alkaline medium although this is not necessary for successfulpractice of the present invention.

The reaction need be carried out for a time sufficient for reaction tooccur. With many reactant combinations, at least a partial reactionoccurs almost instantly. Thus, the particular reaction time is a matterof choice. However, the longer the reaction time the greater the extentof reaction. As a practical matter, a significant amount of product isobtained with a reaction time of about one day, and generally a reactiontime of about 3 to 7 days is advantageous.

After the reaction has been completed (or terminated), the pyrochloreoxide precipitate is displaced from the alkaline medium by water.Typically this displacement is accomplished by allowing the solidprecipitate to settle in the alkaline reaction medium. The majority ofthe alkaline supernatant liquid is then removed by decanting orpipetting leaving just enough liquid so that the precipitate remainssubmerged. The reaction beaker is then refilled with water, theprecipitate re-slurried and allowed to settle. Most of the liquid ispipetted off and replaced with water. This procedure is repeated severaltimes (typically three) with the precipitate being kept completelysubmerged throughout the entire procedure. The number of necessaryliquid displacements can be determined by measuring the pH of thedecanted or pipetted supernatant liquid. When the pH has returned toessentially a neutral level (i.e., pH of about 7.0), no furtherdisplacements are required.

The resultant slurry of water and pyrochlore precipitate is thenatomized to form a fine spray. Atomization may be effected in anysuitable apparatus for forming fine sprays. Typically, atomization isachieved with a hydraulic spray nozzle or a pneumatic atomizing nozzle.The pneumatic atomizing nozzles, which use compressed air to effectspray particle separation, produce the very finest droplets and aretherefore preferred. A precise description of exactly what nozzles aresuitable is difficult since a number of parameters which affectatomization (e.g., flow rate, pressure, orifice size, etc.) may bevaried on any particular nozzle. It is expected that those skilled inthe art are cognizant of the effects of these parameters and of therelative atomization capabilities of various spray nozzles. In general,any atomization nozzle may be used, but the extent of agglomeration inthe product powder will be minimized by using those nozzles whichproduce the finest droplet size. Efficient atomization can rarely beobtained with air pressures less than 10 p.s.i.

For hydraulic spray nozzles, the factors providing finer spray dropletsinclude small capacities (in gallons per hour), higher sprayingpressures and hollow cone spray patterns. Conversely, coarser spraydroplets are produced by larger capacities, lower spraying pressures andfull cone spray patterns.

The finest atomization is most easily obtained with air atomizingnozzles because compressed air is used to separate the liquid into fineparticles. At any given liquid pressure, a change in air pressure notonly affects capacity but also spray droplet size. Increasing airpressure at any liquid pressure decreases liquid droplet size, anddecreasing air pressure increases liquid droplet size. Representative ofthe type of spray nozzles which are found to be suitable are the 1/4 Jseries air atomization nozzles manufactured by Spraying Systems Company,Wheaton, Ill.

The resulting spray is then frozen by contact with a freezing medium(e.g., liquid nitrogen) to yield a finely divided frozen powder whichcomprises an intimate mixture of pyrochlore compound and ice slurried inthe freezing medium. Suitable freezing medium include hexane chilled bydry ice and acetone and dichlorodifluoromethane (Freon 12, E. I. duPontde Nemours & Co.).

The frozen slurry is then freeze dried (i.e., the ice is removed fromthe slurry by sublimination). Suitable methods of freeze drying aredescribed in F. J. Schnettler, F. R. Monforte, & W. W. Rhodes, Scienceof Ceramics, Vol. 4, pp. 79-90, The British Ceramic Society,Stoke-on-Trent, England (1968), the entire disclosure of which isincorporated herein by reference.

The reaction product includes one or more of the pyrochlore compounds ofthe formula (1) above. When preferred amounts of reactants are employed,compounds of formula (1) may be obtained wherein 0<x≦1.0. Thus, amongthe compounds obtained are:

    Pb.sub.2 [Ru.sub.2-x Pb.sub.x ]O.sub.7-y                   (2)

    Pb.sub.2 [Ir.sub.2-x Pb.sub.x ]O.sub.7-y                   (3)

    PbBi[Ru.sub.2-x Pb.sub.x ]O.sub.7-y                        (4)

    PbBi[Ir.sub.2-x Pb.sub.x ]O.sub.7-y                        (5)

    Pb.sub.a Bi.sub.b [Ru.sub.2-x Pb.sub.x ]O.sub.7-y          (6)

    Pb.sub.2 [Ru.sub.2-x (Pb.sub.c Bi.sub.d).sub.x ]O.sub.7-y  (7)

and the like, wherein x and y are as defined, and wherein a+b=2 andc+d=x. Also, included are the bismuth-rich counterparts to the foregoingand other variations within the scope of formula (1) which should now beapparent to the artisan. If desired, various post treatments may beemployed (e.g. heat treatments to improve the crystallinity of theproduct and/or washing in various media in order to leach out anyunreacted metal species). The above pyrochlores produced by the methodof the present invention have essentially the same surface area as apowder prepared by the method disclosed in U.S. Pat. Nos. 4,129,525 and4,176,094 (e.g., 50-200 m² /g). However, the bulk density is aboutone-tenth of the prior methods, which indicates reduced agglomeration.

A critical aspect of this invention is displacing the alkaline reactionmedium with water without exposing the pyrochlore precipitate to air. Ifnot kept submerged in liquid continually, the precipitate agglomeratesirreversibly which results in a higher bulk density.

The reduced agglomerization in the powders prepared according to thepresent invention is reflected in superior electrocatalytic activity.FIG. 3 shows oxygen electro-reduction activity for two ruthenatepyrochlores which are identical except that one was dried conventionallywhile the other was atomized and freeze dried according to the presentinvention. The atomized and freeze dried product shows significantlybetter activity.

The present invention will be more fully understood by reference to thefollowing examples which are presented for illustrative purposes only,and should not be construed to limit the claims appended hereto:

EXAMPLE 1

A bismuth-rich pyrochlore, e.g., Bi₂ [Ru_(2-x) Bi]O_(7-y), is preparedas follows:

Bi(NO₃)₃.5H₂ O and Ru(NO₃)₃ are combined in aqueous solution in anapproximately 1.2:1.0 molar ratio of bismuth to ruthenium. That is,about 28.81 grams of Bi(NO₃)₃.5H₂ O and about 55.60 grams of Ru(NO₃)₃aqueous solution (9% by weight Ru) are added to 130 ml of concentrated(15.7 N) HNO₃ and 250 ml of water. The aqueous solution of bismuth andruthenium is then added, with stirring, to 500 ml of 3 M KOH at 75° C.Precipitation of a solid occurs immediately. At this point, sufficientKOH and water are added to adjust the pH to 13.5 and the total volume to2000 ml. The beaker is kept covered and the slurry is sparged withoxygen. The reaction is carried out at 75° C., with stirring, forapproximately 7 days. The reaction mixture is cooled, and theprecipitate allowed to settle. Most of the alkaline medium is removedwith a pipette, leaving just enough liquid so that the precipitateremains submerged. The beaker is then refilled with water, theprecipitate re-slurried and allowed to settle. Most of the liquid ispipetted off and replaced with water. This procedure was repeated twice,with the precipitate being kept completely submerged throughout theentire procedure.

Finally, most of the water is pipetted off one last time so that a thickslurry remains. The slurry is then sprayed through an air atomizationnozzle into a covered beaker containing liquid nitrogen. The nozzle,manufactured by Spraying Systems Co., Wheaton, Ill., is model 1/4 JCOfitted with fluid cap #2850 (liquid orifice diameter of 0.028") and aircap #70. The nozzle is pressurized by 20 p.s.i. of air. The resultingslurry of liquid nitrogen and finely divided frozen powder is thenfreeze dried. X-ray diffraction shows that the reacted product is acrystalline material exhibiting the pyrochlore crystal structure. Thebismuth to ruthenium ratio, as determined experimentally by X-rayfluorescence, is 1.44:1.0. The formula for the pyrochlore may thereforebe expressed as Bi₂ [Ru₁.64 Bi₀.36 ]O_(7-y). The surface area measuredby the BET N₂ adsorption method is 120 m² /g. The bulk (tap) density ofthe powder is measured to be 0.1 g/cc.

EXAMPLE 2

A bismuth-rich pyrochlore, e.g., Bi₂ Ru_(2-x) Bi_(x) O_(7-y), isprepared as follows:

Bi(NO₃)₃.5H₂ O and Ru(NO₃)₃ are combined in aqueous solution in anapproximately 1.2:1.0 molar ratio of bismuth to ruthenium. That is,about 14.396 grams of Bi(NO₃)₃.5H₂ O and about 31.250 grams of Ru(NO₃)₃aqueous solution (8% by weight Ru) are added to 50 ml conc.HNO₃ and 100ml water. This aqueous solution of bismuth and ruthenium is then added,with stirring, to 250 ml of 3MKOH at 75° C. Precipitation of a solidoccurs immediately. Sufficient KOH and water are then added to adjustthe pH to 13.5 and the total volume to 1000 ml. The beaker is keptcovered and the slurry is sparged with oxygen. The reaction is carriedout at 75° C., with stirring, for approximately 7 days. The reactionmixture is cooled and the precipitate allowed to settle.

About one third of the wet precipitate is scooped out with a spatula,placed in a container, and immersed with liquid nitrogen. The frozensample is then freeze dried. This sample is designated 2-A.

The remaining two thirds of the precipitate is separated by vacuumfiltration, washed with water and dried in a conventional manner. Thissample is designated as 2-B. Approximately half of this conventionallydried sample (2-B) is then re-slurried in water and allowed to settle.Most of the water is then removed by vacuum pipetting, leaving a thickslurry. This slurry is then sprayed through an air atomization nozzle asdescribed in Example 1. This sample is designated as 2-C.

X-ray diffraction shows that each sample is a crystalline materialexhibiting the pyrochlore crystal structure. The bismuth to rutheniumratio, as determined experimentally by X-ray fluorescence, is 1.76:1.0.The formula for each pyrochlore sample may therefore be expressed as Bi₂[Ru₁.45 Bi₀.55 ]O_(7-y).

A comparison of the properties of the samples discussed in Examples 1and 2 is given in Table I.

                  TABLE I                                                         ______________________________________                                                                    Sur-                                                                          face    Bulk                                                                  Area    Density                                   SAMPLE        Composition   (m.sup.2 /g)                                                                          (g/cc)                                    ______________________________________                                        Example 1     Bi.sub.2 [Ru.sub.1.64 Bi.sub..36 ].sub.7-y                                                  120     0.1                                       Atomized/Freeze Dried                                                         Sample 2-A    Bi.sub.2 [Ru.sub.1.45 Bi.sub..55 ]O.sub.7-y                                                 162     0.4                                       Freeze Dried                                                                  Sample 2-B    Bi.sub.2 [Ru.sub.1.45 Bi.sub..55 ]O.sub.7-y                                                 137     1.3                                       Conventionally Dried                                                          Sample 2-C    Bi.sub.2 [Ru.sub.1.45 Bi.sub..55 ]O.sub.7-y                                                 141     1.1                                       Conventionally Dried,                                                         Re-Slurried,                                                                  Atomized/Freeze Dried                                                         ______________________________________                                    

While the above-described variations in powder preparation conditions donot introduce significant differences in surface area, such variationshave a profound effect on bulk density (i.e., agglomeration). As shownin Table I, the lowest density (and lowest degree of agglomeration), 0.1g/cc, is obtained when the atomization/freeze drying of the precipitateis employed. If the powder is merely freeze dried without first beingsubjected to atomization, the bulk density is 0.4 g/cc. In contrast, aconventionally dried sample has a bulk density of 1.3 g/cc. Finally, ifa conventionally dried sample is re-slurried and then atomized andfreeze dried, the resulting bulk density (1.1 g/cc) shows littleimprovement over a conventionally dried sample. This result demonstratesthat a precipitate will suffer irreversible agglomeration when exposedto air, rather than being continually submerged in a liquid.

What is claimed is:
 1. A method of preparing compounds having theformula:

    A.sub.2 [B.sub.2-x A.sub.x ]O.sub.7-y

wherein A is selected from the group consisting of lead, bismuth andmixtures thereof, wherein B is selected from the group consisting ofruthenium, iridium and mixtures thereof, wherein x is a value such that0<x≦1.0 and y is a value such that 0≦y≦1, comprising: (a) reacting Acations and B cations from an aqueous solution source of these cationsin a liquid alkaline medium having a pH of at least about 12.0 in thepresence of an oxygen source at a temperature below about 200° C. for asufficient time to form a pyrochlore precipitate, (b) displacing theliquid alkaline medium with water to form a slurry comprising water andpyrochlore precipitate, thereby avoiding contact of the precipitate thusformed with air; (c) atomizing the slurry thus formed to produce a finespray, (d) contacting the spray thus formed with a freezing medium for aperiod of time sufficient to form a frozen powder comprising an intimatemixture of pyrochlore compound and ice slurried in the freezing medium,(e) freeze drying the frozen powder thus formed to recover a pyrochlorecompound of reduced agglomeration relative to that obtained in theabsence of steps (c)-(e).
 2. The method of claim 1 wherein said aqueoussolution source contains A source material selected from the groupconsisting of lead nitrate, lead oxide, lead chloride, lead acetate,lead carbonate, lead citrate, lead oxalate, bismuth nitrate, bismuthoxide, bismuth chloride, bismuth oxalate, bismuth oxychloride andmixtures thereof and B source material selected from the groupconsisting of ruthenium chloride, ruthenium nitrate, ruthenium nitrosylnitrate, iridium chloride, iridium hydroxide and iridium oxalic acid. 3.The method of claim 2 wherein said liquid alkaline medium is an aqueousbasic solution of alkali metal hydroxide.
 4. The method of claim 3wherein said aqueous basic solution contains a base selected from thegroup consisting of sodium hydroxide, rubidium hydroxide, cesiumhydroxide, potassium hydroxide and mixtures thereof.
 5. The method ofclaim 4 wherein said reacting is performed within the temperature rangeof about 10° C. to about 100° C.
 6. The method of claim 5 wherein saidpH is within the range of about 13 to
 14. 7. The method of claim 6wherein said reacting is performed within the temperature range of about50° C. to about 80° C.
 8. The method of claim 1 wherein A is lead. 9.The method of claim 8 wherein said aqueous solution source contains as Asource material lead nitrate and contains B source material selectedfrom the group consisting of ruthenium nitrate and iridium chloride. 10.The method of claim 9 wherein said liquid alkaline medium is an aqueousbasic solution of alkali metal hydroxide.
 11. The method of claim 10wherein said aqueous basic solution contains a base selected from thegroup consisting of sodium hydroxide, rubidium hydroxide, cesiumhydroxide, potassium hydroxide and mixtures thereof.
 12. The method ofclaim 11 wherein said reacting is performed within the temperature rangeof about 10° C. to about 100° C.
 13. The method of claim 12 wherein saidpH is within the range of about 13 to about
 14. 14. The method of claim13, wherein said reacting is performed within the temperature range ofabout 50° C. to about 80° C.
 15. The method of claim 1 wherein A isbismuth.
 16. The method of claim 15 wherein said aqueous solution sourcecontains as A source bismuth nitrate and contains B source materialselected from the group consisting of ruthenium nitrate and iridiumchloride.
 17. The method of claim 16 wherein said liquid alkaline mediumis an aqueous basic solution of alkali metal hydroxide.
 18. The methodof claim 17 wherein said aqueous basic solution contains a base selectedfrom the group consisting of sodium hydroxide, rubidium hydroxide,cesium hydroxide, potassium hydroxide and mixtures thereof.
 19. Themethod of claim 18 wherein said reacting is performed within thetemperature range of about 10° C. to about 100° C.
 20. The method ofclaim 19 wherein said pH is within the range of about 13 to about 14.21. The method of claim 20 wherein said reacting is performed within thetemperature range of about 50° C. to about 80° C.